Air Quality & Climate

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Notes

Composition of Air

Clean, dry air is approximately **78% nitrogen** and **21% oxygen**.
The remaining 1% includes **carbon dioxide** and **noble gases** (e.g., argon).
This composition has remained stable for about 200 million years.

Air Pollutants: Sources & Effects

**Carbon dioxide (CO₂)**: from **complete combustion** of fossil fuels; causes **global warming**.
**Carbon monoxide (CO)**: from **incomplete combustion**; **toxic** – binds to haemoglobin, preventing oxygen transport.
**Particulates (soot)**: from incomplete combustion; cause **respiratory problems** and **cancer**.
**Methane (CH₄)**: from animal digestion, decomposition, and landfill; a potent **greenhouse gas**.
**Oxides of nitrogen (NOₓ)**: formed at high temperatures in car engines; cause **acid rain** and **photochemical smog**.
**Sulfur dioxide (SO₂)**: from burning sulfur-containing fossil fuels; causes **acid rain**.

Complete vs Incomplete Combustion

**Complete combustion** (excess O₂): produces CO₂ and H₂O only.
**Incomplete combustion** (limited O₂): produces CO, H₂O, and sometimes **carbon (soot)**.
Example (complete): CH₄ + 2O₂ → CO₂ + 2H₂O.
Example (incomplete): 2CH₄ + 3O₂ → 2CO + 4H₂O.

Greenhouse Effect & Global Warming

The **greenhouse effect** is natural: greenhouse gases (CO₂, CH₄) trap thermal energy, keeping Earth warm.
**Enhanced greenhouse effect**: increased greenhouse gas concentrations trap more heat, causing **global warming**.
Consequences: **climate change**, **rising sea levels** (glacier melt), **species extinction**, **habitat loss**, **disease spread**.

Reducing Climate Change

Reduce CO₂ emissions: use **renewable energy** (solar, wind), **fuel-efficient vehicles**, **public transport**, **recycling**, and **plant trees**.
Reduce methane emissions: **reduce livestock farming** (methane from digestion).
Reduce deforestation and promote **reforestation** to absorb CO₂ via photosynthesis.

Reducing Acid Rain

Acid rain is caused by **SO₂** and **NOₓ** dissolving in rainwater.
Use **low-sulfur fuels** to reduce SO₂ emissions.
**Flue gas desulfurisation**: spray waste gases with lime slurry (CaO/CaCO₃) to remove 90–95% SO₂.
**Catalytic converters** in vehicles remove NOₓ by reducing them to N₂.

Catalytic Converters

Contain **platinum** and **rhodium** catalysts in a **honeycomb** to increase surface area.
**Oxidise** CO to CO₂: 2CO + O₂ → 2CO₂.
**Reduce** NOₓ to N₂: 2NO → N₂ + O₂; 2NO₂ → N₂ + 2O₂.
Overall reaction: 2NO + 2CO → N₂ + 2CO₂.
Unburned hydrocarbons are oxidised to CO₂ and H₂O.

Photosynthesis

**Endothermic** reaction using light energy absorbed by **chlorophyll**.
Reactants: **carbon dioxide + water** → Products: **glucose + oxygen**.
Word equation: carbon dioxide + water → glucose + oxygen.
Symbol equation (extended): 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Conditions: **light** and **chlorophyll**.

Particle arrangement in solid, liquid, and gas states. Useful for understanding combustion and pollutant formation.

Practice questions

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1.What is the approximate percentage of nitrogen in clean, dry air?
Easy
- A78%
- B21%
- C50%
- D1%
2.Which gas is produced by the complete combustion of methane?
Easy
- ACarbon dioxide
- BCarbon monoxide
- CSulfur dioxide
- DNitrogen dioxide
3.Which of the following is a greenhouse gas?
Medium
- AMethane
- BOxygen
- CNitrogen
- DArgon
4.What is the main cause of acid rain?
Medium
- AOxides of nitrogen and sulfur dioxide
- BCarbon dioxide and methane
- CCarbon monoxide and particulates
- DOzone and water vapor
5.In a catalytic converter, what is the function of the honeycomb structure?
Hard
- ATo increase the surface area for catalytic reactions
- BTo filter out particulates
- CTo cool the exhaust gases
- DTo store unburnt fuel
6.Which of the following is a product of photosynthesis?
Medium
- AGlucose
- BCarbon dioxide
- CWater
- DChlorophyll
7.Which process produces carbon monoxide?
Easy
- AIncomplete combustion of fuels
- BComplete combustion of fuels
- CPhotosynthesis
- DRespiration
8.What is the main strategy to reduce sulfur dioxide emissions from power stations?
Medium
- AFlue gas desulfurisation
- BUsing catalytic converters
- CPlanting more trees
- DReducing livestock farming

Slides

Composition of Clean, Dry Air

The composition of the atmosphere has been stable for about 200 million years.
Clean, dry air is approximately 78% nitrogen and 21% oxygen.
The remaining 1% includes carbon dioxide and trace noble gases (e.g., argon).

What is the approximate percentage of oxygen in clean, dry air?

78%
21%
1%
0.04%

Air Pollutants and Their Sources

Carbon dioxide (CO₂): from complete combustion of fossil fuels.
Carbon monoxide (CO): from incomplete combustion of fuels.
Particulates (soot): from incomplete combustion of fuels.
Methane (CH₄): from animal digestion, decomposition, and landfill sites.
Oxides of nitrogen (NOₓ): formed in high-temperature engines and furnaces.
Sulfur dioxide (SO₂): from burning fossil fuels that contain sulfur.

Which of these are sources of carbon monoxide?

Complete combustion of methane
Incomplete combustion of gasoline
Reaction of nitrogen and oxygen in car engines
Incomplete combustion of methane

Adverse Effects of Pollutants

CO₂ and methane contribute to global warming and climate change.
CO is toxic – it binds to haemoglobin, reducing oxygen transport.
Particulates cause respiratory problems and cancer.
NOₓ and SO₂ cause acid rain, damaging buildings, crops, and aquatic life.
NOₓ also produces photochemical smog and irritates lungs.

Which two pollutants are the main causes of acid rain?

Carbon monoxide and particulates
Oxides of nitrogen and sulfur dioxide
Methane and carbon dioxide
Oxygen and nitrogen

The Greenhouse Effect

The Sun emits radiation that reaches Earth. Some is reflected, some absorbed and re-emitted as infrared (thermal) energy.
Greenhouse gases (CO₂, CH₄) absorb some of this infrared radiation and re-emit it in all directions, trapping heat.
This natural greenhouse effect keeps Earth warm enough for life.
Human activities increase greenhouse gas concentrations, enhancing the effect and causing global warming.

The two main greenhouse gases are carbon dioxide and ____.

Consequences of Global Warming

Climate change: more extreme weather, droughts, and storms.
Rising sea levels due to melting glaciers and ice caps, causing flooding in low-lying areas.
Extinction of species as habitats are destroyed.
Migration of species to more habitable areas.
Spread of diseases due to warmer climates.

Which of these are consequences of global warming?

Increased acid rain
Rising sea levels
More frequent droughts
Reduced carbon dioxide levels

Reducing Climate Change

Reduce CO₂ emissions: use renewable energy (solar, wind), fuel-efficient vehicles, public transport, and recycle.
Reduce deforestation and plant more trees (photosynthesis removes CO₂).
Reduce methane emissions: cut livestock farming.

Which strategy helps reduce carbon dioxide levels in the atmosphere?

Increasing livestock farming
Planting trees
Using more fossil fuels
Deforestation

Reducing Acid Rain

Acid rain is caused by SO₂ and NOₓ.
Use low-sulfur fuels to reduce SO₂ emissions.
Flue gas desulfurisation (FGD) in power stations: spray lime slurry to remove SO₂.
Catalytic converters in vehicles remove NOₓ and CO.

In a catalytic converter, carbon monoxide is oxidised to ____ and oxides of nitrogen are reduced to ____.

Photosynthesis

Photosynthesis is an endothermic reaction that uses light energy.
Reactants: carbon dioxide and water.
Products: glucose and oxygen.
Conditions required: chlorophyll (in chloroplasts) and light energy.
Word equation: carbon dioxide + water → glucose + oxygen.

Complete the word equation for photosynthesis: carbon dioxide + water → ____ + ____

Catalytic Converters (Extended)

Catalytic converters contain transition metal catalysts (platinum, rhodium) in a honeycomb structure to increase surface area.
They remove CO, NOₓ, and unburned hydrocarbons from exhaust.
CO is oxidised to CO₂: 2CO + O₂ → 2CO₂.
NOₓ is reduced to N₂: 2NO → N₂ + O₂; 2NO₂ → N₂ + 2O₂.
Overall: 2NO + 2CO → N₂ + 2CO₂.
Unburned hydrocarbons are oxidised to CO₂ and H₂O.

What is the overall reaction in a catalytic converter between nitrogen monoxide and carbon monoxide?

2NO + 2CO → N₂ + 2CO₂
2NO + CO → N₂O + CO₂
NO + CO → N + CO₂
2NO₂ + 4CO → N₂ + 4CO₂

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